This invention relates to a method for controlling an internal combustion engine mounted in a vehicle having an air conditioning system, and more particularly, to a method for adjusting the output power delivered by the engine to compensate for changes in engine loading induced by a variable capacity type air conditioning compressor.
The idling rotational speed of an internal combustion engine is customarily controlled in a closed-loop fashion, by regulating the amount of output power delivered by the engine, in response to a difference between actual engine speed and a desired target idling speed. Any of several standard power control mechanisms may be employed to regulate engine output power for this purpose. For example, it is well known that idle speed can be controlled by regulating an engine control parameter such as ignition spark timing, the amount of fuel supplied to the engine, or the quantity of air inducted into the engine.
In modern computer engine control systems, the engine parameter selected for use in regulating engine output power is normally controlled by a base idle variable retained in computer memory. A change in the value of this base idle variable produces a corresponding change in setting of the engine power control mechanism, which in turn varies the engine parameter being controlled and the output power delivered by the engine. The value of the base idle variable is continuously updated in response to the closed-loop idle control routine, and its assigned value corresponds to the current estimate for the engine control parameter, that will bring the engine to the desired target idling speed, under the present and/or anticipated engine loading conditions. As the engine warms up from a cold start, the base idle variable is usually decreased in value, as a function of the engine coolant temperature, to reduce fuel consumption as the risk of stalling diminishes. It is also common practice to increase the value of the base idle variable by fixed amounts to increase engine output power, in anticipation of significant loads being placed on the engine, such as when a vehicle air conditioner is switched on.
With traditional automobile air conditioning systems, the refrigerant pressure must be regulated to prevent it from becoming too great and rupturing the system. This is normally accomplished by cycling the clutch of the air conditioning compressor on and off, to keep the refrigerant pressure within acceptable limits. This cycling of the compressor results in large, and substantially constant load transients on the vehicle engine. Because these load transients occur very rapidly, the closed-loop idle control is not able to respond rapidly enough to compensate for the changes in loading. This results in large sags and surges in the engine idling speed, when the air conditioning load is applied to and removed from the engine. Thus, it is customary to add or subtract a fixed amount to or from the stored base idle variable, just prior to the engaging or disengaging of the air conditioning compressor clutch, to adjust engine output power in anticipation of the increased or decreased load on the engine, in order to maintain an acceptable idling engine speed.
Recently, a new variable capacity type air conditioning compressor has become commercially available for use in automobiles. This compressor includes a mechanism, whereby its capacity can be varied to adjust the refrigerant pressure. The compressor is designed to minimize its capacity upon starting, and then automatically vary its running capacity to regulate the pressure of the refrigerant to achieve a substantially constant inlet refrigerant pressure. When an air conditioning system employing this type of compressor is switched on, the compressor clutch is engaged and the compressor runs continuously, rather than being cycled on and off. When the ambient temperature is relatively high, the compressor operates at a higher capacity, inducing a relatively large load on the engine, due to the large thermal load on the air conditioning system. On the other hand, when the ambient temperature is low, the thermal load is reduced, and the compressor operates at a lower capacity, thereby reducing its load on the engine.
Because the above described variable capacity compressor induces a variable rather than a fixed engine load, the conventional control technique of adding or subtracting a fixed amount, to compensate the base idle variable for air conditioner loading, can not be used. If the engine is operating at idle, and the compressor has a low starting torque, the addition of too large a fixed amount to the base idle variable will produce an unacceptable surge in engine speed. When the compressor has a higher starting torque, the load will be larger than anticipated by the fixed amount, and engine rotational speed will sag when the load is applied, with possible engine stalling.
An additional problem is encountered when the engine of an automobile equipped with this type of variable capacity air conditioning compressor is operated off-idle. During off-idle engine operation, the air flow to the vehicle components in the engine compartment increases. This increases the capacity of the condenser in the air conditioning system, due to the improved transfer of heat. To compensate for the increased condenser capacity and maintain the refrigerant pressure at the proper level, the compressor reduces its capacity, which in turn reduces the load on the engine. This reduction in air conditioner loading with increased air flow, results in a "sail-on" feeling to the driver, when the engine throttle is closed for a coasting condition, and too great an engine speed, when the engine is returned to idle. This occurs because the closed-loop idle control system is inoperative, when the engine is operated off-idle, and consequently, engine output power is not adjusted to compensate for the reduced loading of the air conditioning system.
Therefore, a need exists for a method of adjusting the output power delivered by an engine, to compensate for changing loading conditions induced by the above described variable capacity air conditioning compressor.